Conventional bicycle parts, such as a freewheel or a free type gear crank means, employ a unidirectional rotary transmission mechanism which comprises, as shown in the freewheel in FIG. 4, a cylindrical driving member DV having at its inner periphery ratchet teeth A. The mechanism also includes a driven member DN provided with first and second transmitting pawls B and C having engaging portions B1 and C1 engageable with the ratchet teeth A and semicircular bases B2 and C2 and supported to the driven member DN to be capable of rising or falling through the bases B2 and C2 respectively. The mechanism also includes a C-shaped pawl spring S having a pair of first and second free ends S1 and S2 and biasing the transmitting pawls B and C toward the ratchet teeth A respectively. The driven member DN has a projection E for locking the pawl spring S, so that the driving member DV, when rotating in the normal direction, transmits its driving force to the driven member DN through the ratchet teeth A and transmitting pawls B and C, thereby rotating the driven member DN integrally with the driving member DV. On the other hand, when the driving member DV rotates in the reverse direction, the transmitting pawls B and C fall against the pawl spring S and the engaging portions B1 and C1 ride over the ratchet teeth A, whereby the transmitting pawls B and C disengage therefrom. As a result, the driving member DV freely rotates with respect to the driven member DN.
However, the first and second transmitting pawls B and C in the conventional unidirectional rotary transmission mechanism of FIG. 4 are disposed symmetrically with respect to a straight line X passing through the center of an interval between the first and second free ends S1 and S2 of the pawl spring S and the axis of the driven member DN. This yields a different in the distances between the engaging portions of the pawl spring S engageable with the transmitting pawls B and C and the free S1 and S2. As a result, a spring force of the spring S when the second transmitting pawl C falls against the pawl spring S is smaller than that when the first transmitting pawl B falls against the same.
When the first and second transmitting pawls B and C fall against the spring force of the pawl spring S, the spring S engages with the edges of the bases of B2 and C2 of the transmitting pawls B and C and these engaging portions are urged radially outwardly of the driven member DN so as to cause pawls B and C respectively to fall down toward driven member DN. The base B2 of the first transmitting pawl B is positioned farther away from first free end S1 of the spring S than engaging portion B1 of the first pawl B. Base C2 of the second transmitting pawl C is positioned closer to second free end S2 of the spring S than engaging portion C1 of the second pawl C. In other words, a length between the first free end S1 and the engaging portion of the pawl spring S engageable with the first transmitting pawl B is larger than that between the second free-end S2 and the engaging portion of the pawl spring S engageable with the second transmitting pawl C, whereby the spring force acting on the second transmitting pawl C becomes smaller than that on the first transmitting pawl B.
The spring constant of the pawl spring S has hitherto been set to apply to the second transmitting pawl C a spring force sufficient to raise it, so that the first transmitting pawl B, which is originally subjected to a large spring force, will be subjected to an even larger spring force. Accordingly, and increase in the spring force applied to the first transmitting pawl B enlarges a rotational resistance of the driving member when freely rotating, thereby creating a problem in that not only does the rotation efficiency of the driving force deteriorate but also the transmitting pawls, which ride over each ratchet tooth, strike the bottom thereof to generate large noises.